Threshold Corporation is known primarily for their amplifiers, but,
like many companies, they are branching out into other products. For a couple of years
now, they have been preparing to market (import) electrostatic loudspeakers (ESLs)
manufactured in Europe.
It has been a long slow process of refining the original product,
with lighter weight polymer membranes in between the stators, and also
designing a method so that the usual input coupling capacitor can be eliminated. The
ES-500s, along with other models including the DCI (Direct Coupled Input), are the result
of this research (click here to see
description of the DCI speaker in the 1998 CES Report). ESLs consist of two metal "Stators" to which the audio signal is applied after having passed through a transformer to boost the voltage. A thin plastic membrane (diapragm), coated with something like graphite, is suspended between the stators, and a high negative potential ("bias voltage") is applied to the membrane. The voltage from the audio signal causes the membrane to be repelled by, or attracted to the stators, and sound emerges through openings in the stators.

Electrostatic speakers are very easy to make, but they are very
difficult to engineer properly. Although
the basic design of having a statically charged (high voltage) plastic membrane suspended
in between two perforated metal plates (stators) carrying the audio signal, is
straightforward, the final design varies widely among the numerous manufacturers who
market them. Most are bowed outward towards the listener, because ESLs tend to
"beam", meaning that the lateral dispersion is narrow. Beaming results in a
smaller sweet spot, so by curving the stators outward, the dispersion is increased. The
Threshold series diverge from the usual bowed design in that they are flat across the
front. Secondly, instead of perforated stators, the Thresholds have thin metal vertical
rods 1/16" in diameter, spaced 1/16" apart, and held in position by horizontal
retainers placed every 4 inches (see photo at right). The stators are 60" high by 12
1/2" wide, giving 750 inch2 of active panel surface.

For the ES-500s, I found the lateral dispersion to be about 100,
which is extremely narrow. When I sat within 8 feet of the speakers, I had to either put
them close together, or toe them in quite a bit. The alternative is to move the listening
position back to about 12 feet, which is probably not feasible in many listening rooms,
especially when you take into account that they have to be a couple of feet away from rear
and side walls. So, these speakers need a big room. However, that is typical for ESLs in
general. (You also need an AC wall outlet nearby to supply the power for the static voltage on the membrane.)

The use of round metal rods instead of the usual perforated metal
plates would supposedly reduce the effects of diffraction since there are only the
vertical edges (except for the retainers) of the rods, instead of vertical and horizontal
edges of small holes in the more conventional designs. Secondly, perforated plates have
rounded edges but flat regions in between the holes, while the rods are rounded throughout
the surface. Lastly, there would be more open area in the stators when using vertical rods instead of perforations in a metal plate, allowing more sound to
come through. The complexity in manufacturing the panels with all the rods straight and in line with the rods on the opposite panel (front and back) is undoubtedly the main reason these speakers are so expensive.

By using a flat panel instead of a curved one, phase relationships
on the horizontal plane are retained. That is, the waveform as it emerges from the edge of
the membrane is in phase with the waveform as it emerges from the center of the membrane.
On a curved panel, the waveform from the edge of the membrane is behind the waveform
emerging from the center, since the center is situated more forward with respect to the
listening position. Of course, one gives up the lateral dispersion by using a flat panel.

The ES-500s are very difficult to place due not only to their
dipolar nature, but their narrow dispersion. In our lab, I was able to situtate them about
6 feet apart, slightly toed in, and with the listening position 12 feet back. Even then,
the sweet spot was not very big.

Because the surface area of the panels is so large, the ES-500s
could move a significant amount of air even at low frequencies. However, I heard
significant resonance when the music was deep and powerful, such as with the Telarc CD of
Copland's "Fanfare for the Common Man". In the room response tests, 31.5 Hz sine
waves made the panel resonate. So, I used an AudioControl Equalizer to rolloff the signal
below 63 Hz, and sent the < 60 Hz signal to a pair of Velodyne FSR-18 subwoofers. This
made a great deal of difference. There was no longer any resonance, and even though the
ES-500s are full range, the FSR-18s are much more capable in the deep end. For general
listening tests, I did not use the subwoofers, and for most music at modest listeing
levels, they would not really be necessary, but I would recommend them for a complete
setup.

Male and female voices, in particular, were as natural as I have
ever heard them. In symphonic music, violins, brass, and piano were extremely detailed,
indicating that there might just be something to the use of completely rounded surfaces in
the stators. Compared to my ribbon speakers, the ES-500s were much smoother on the high
end. Paul Knutson (a new Secrets writer whose first review will be published soon) felt
they might be a little too smooth, but I liked the effect very much. The overall
impression is, like most dipolar panels, one of large diffusion. The instruments are not
focused in the soundstage, but come from the general direction of "over there".

As you can see, the room response is all over the place. That's one
of the problems with dipolar speakers. Nevertheless, they sounded very good. The response
rolls off at 15 kHz, indicating why Paul Knutson (who has younger ears with better high
frequency hearing) felt they were too smooth at the high end.

At $11,000 a pair, the ES-500s are steep. Because of their narrow
dispersion, I don't consider them to be contenders in home theater applications. However,
as ESLs go, they are really something. Since the import status is not finalized yet, you
may not see them for awhile, but if you eventually come across them sitting in the window
display (they are really attractive) of a high performance dealer, go in and make the
sales force give you a demo.